The present invention relates to a Blow Out Preventer (BOP) assembly and a Lower Riser Package (LRP) on an oil or gas field well. The invention is particularly useful in Through Tubing Rotary Drilling (TTRD).
TTRD is a relatively new industry requirement which allows operators to drill through existing production trees and existing well bores to reach additional hydrocarbons in an oil bearing formation. After a period of oil production in a wellhead, the oil bearing formation below the wellhead becomes depleted and fluids or gases may not flow into the well bore and then through the tubing for production. TTRD allows an operator to enter a well and drill a side track into the oil bearing formation without having to remove the tubing and associated production equipment such as an xmas tree from the wellhead.
When drilling down to create the well hole it is necessary to use a BOP assembly. However, in TTRD where an existing wellhead and xmas tree are being drilled through, conventional drilling or a dedicated TTRD BOP stack would be very costly.
A BOP assembly is a multi closure safety device which is connected to the top of a drilled, and often partially cased, hole. The accessible top end of the casing is terminated using a casing spool or wellhead housing to which the BOP assembly is connected and sealed.
The wellhead and BOP stack (the section in which rams are provided) must be able to contain fluids at a pressure rating in excess of any formation pressures that are anticipated when drilling or when having to pump into the well to suppress or circulate an uncontrolled pressurised influx of formation fluid or gases. This influx of formation fluid is known as a “kick” and restabilising control of the well by pumping drilling control fluid (known as mud) to suppress the influx or to circulate the influx out is necessary to control the well. An uncontrolled escape of fluid, whether liquid or gas, to the environment is termed a ‘blow out’. A blow out can result in a major leak to the environment which can ignite or explode, jeopardising personnel and equipment in the vicinity, and can result in serious pollution.
Although normal drilling practices provide a liquid hydrostatic pressure barrier for well control, a second safety barrier to contain a kick is provided mechanically by the BOP assembly. The BOP assembly must close and seal on tubular equipment (i.e., pipe, casing or tubing) hung or operated through the BOP assembly and ultimately must be capable of sealing off the well. A general term for a tubular system run into the well is called a string. Wells are typically drilled using a drill string using progressively smaller diameter drill pipes and drill bits. During completion (a term related to bringing a well on stream) or carrying out a workover intervention, various diameter of tubulars, coiled tubing, cable, wireline (including combined bundles) and an assortment of tools are run.
A subsea conventional drilling BOP stack assembly is attached to a wellhead and is provided with a number of pairs of rams to seal around different drill pipe diameters or to shear the drill pipe (in an emergency) and seal the bore. These rams should be rated to perform at pressures in excess of any anticipated well pressures or kick control injection pressures and are typically rated at 10 to 15 kpsi (69-103 MPa). The BOP rams are always used in pairs and when operated, each ram of the pair move towards each other in order to prevent fluid flow through the bore.
An LRP is typically connected to the top of the xmas tree during tree installation or workover and contains a number (usually two) of gate valves and lightweight wireline BOP rams for cutting and/or sealing small diameter workstrings such as wireline, coiled tubing or electrical lines during the completion phase. It should be noted that a drilling BOP stack has more sets of rams (normally a minimum of three sets) compared with the LRP where one set of lightweight BOP rams is usually sufficient. It is worthy of note that drilling BOP rams and associated actuating mechanisms are considerably more expensive than the lightweight BOPs and gate valves.
The term “drilling BOP rams” is well known in the art and is used in this specification to refer to rams that are designed to seal around the drill pipe (sometimes called the drill string) in the event of pressure or a “kick” being encountered during the drilling programme. A different set of rams are usually included in a drilling BOP stack known as shear rams; these rams are used in an emergency and will shear a drill pipe and provide a pressure containing seal against the well.
The term “gate valves” is well known in the art and is used in this specification to refer to valves that are primarily designed as a closure device to provide a pressure seal against gas or liquids (typically oil). Valves have been developed also to cut small diameter work strings (typically, 3.175 mm (0.125″) diameter, wireline and, more recently, small diameter drill pipe—up to 73 mm (2⅞″) diameter and seal afterwards. Valves, however, cannot be substituted entirely for drilling ram type BOPs as they are not designed for sealing around the drill pipe (drill string).
The lightweight BOP rams discussed in the specification are used in LRPs to provide a cutting and sealing facility in preference to using a gate valve; they can also be used to temporarily suspend wireline (or lightweight drill strings)—this cannot be done with a gate valve. Lightweight rams are often preferred to cutting than gate valves as their sealing capabilities (after cutting) are more reliable.
US 2005/0028980 describes a method of suspending, completing or working over a well. The method set out in this document is to meet statutory safety requirements of having a least two barriers in place during the construction or suspension of wells. The method teaches the use of two deep set barriers in the form of plugs positioned at the end of the completion string. As the two deep set barriers are not higher up in the well bore, both of the barriers can remain in place during suspension and completion operation on the well. This means that a drilling BOP stack is not necessary to provide well control and that during operations on the well, only an LRP is required.
WO 03/014604 describes a method and apparatus for the replacement of an entire BOP stack with a gate valve. A gate valve is described that has a slidable gate with a cutting edge that is capable of cutting through production tubing. According to the teaching in this document, this gate valve eliminates the need for a BOP stack and the drilling BOP rams.
U.S. Pat. No. 6,454,015 describes a gate valve which has a gate which is suitable for shearing small diameter wireline. The gate valve does not provide the same function as drilling BOP.
An aim of the invention is to provide a system which allows the conversion of an LRP into a BOP stack assembly so that a separate BOP stack is not required for Through Tubing Rotary Drilling.